Method and apparatus for producing a plasma



Aug. 17, 1965 A. F. CARTER 3,201,635

METHOD AND APPARATUS FOR PRODUCING A PLASMA Filed March 7, 1962 INVENTORARLE/V F.' CARTER BY A ATTO EYS United Sttes atct 3,201,635 Mariano ANDarrana'rris son rnonucuso A PLASMA Arlen F. Carter, Newport News, Va,assignor to the United ates of America as represented by theAdministrator of the National Aeronautics and dpacc Administration FiledMar. 7, 1962, Ser. No. 178,215 3 (Ilaians. (Cl. 3l3--l.56) (Grantedunder Title 35, US. Code (1952), sec. 266) The invention describedherein may be manufactured and used by and for the Government of theUnited States of America for governmental purposes without the paymentof any royalties thereon or therefor.

This invention relates generally to the production of a plasma, and moreparticularly to an apparatus and method for producing a highlyconductive plasma, having an electron concentration on the order ofelectrons per cubic centimeter, and having a homogcnous temperature andpressure distribution.

A highly conductive plasma having a homogenous temperature and pressuredistribution has many, varied, experimental and practical applications.One important application for such a plasma is its use as the workingmedium in hypersonic electromagnetic plasma accelerators. Plasmaaccelerators of this type have a wide range of utility, such asproviding a source of high-speed flow for aerodynamic testing, andproviding a propulsive system for space vehicles. Another application ofsuch a plasma is in the laboratory simulation of the plasma sheath whichis formed around a hypersonic reentry vehicle. Plasma-attenuationexperiments are performed on the laboratory produced plasma in whichtelemetry and microwave signals are transmitted through the plasma todetermine the effect of the plasma on the transmitted signal; therebyaiding in the solution of the problem of transmitting radio-frequencysignals to and from a high altitude hypersonic vehicle.

It is well known that a plasma, or body of ionized gas, may be producedby heating a gaseous material to a temperature whereat the molecules ofthe gas dissociate into positive ions and negative electrons. However,the high degree of ionization required by present day applications hasheretofore not been attainable through chemical processes; and theproduction of a suitable plasma, i.e., a plasma having a concentrationof electrons on the order of 16 per cubic centimeter, through use ofknown electrical arc jet apparatus requires an exorbitant amount ofelectrical power, in the 10 kilowatt range. Further, operation of such ahigh powered are has been found to result in excessive electrodeerosion, and subsequent contamination of the plasma by the erodedelectrode particles.

One object of the present invention is to provide a new and improvedapparatus for producing a highly conductive plasma having a homogenoustemperature and pressure distribution.

Another object of this invention is to provide a novel method ofproducing a plasma having an electron concentration on the order of 10electrons per cubic centimeter.

A further object of this invention is to provide an Patented Aug. 17,1%65 efficient and effective method and apparatus for increasing theconductivity of an electrical arc jet.

A still further object of this invention is to provide a method andapparatus for increasing the electron concentration of an arc jetWithout increasing the electrical power required in operating the arc.

According to the present invention, the foregoing and other objects areattained by providing an electrical arc jet apparatus for producing ahigh temperature gaseous jet and combining therewith a means for seedingthe produced jet with an easily ionizable material in order to increasethe conductivity of the jet. The apparatus comprises a chamber whereinan electrical arc is struck and maintained between a pair of electrodes.A fluid, such as compressed gas, is fed through the arc and heated so asto form a high temperature, gaseous jet. The gaseous jet then passesinto a settling chamber located adjacent said electrodes wherein the jetis seeded and mixed with the ionizing agent. The ionizing agent isstored in liquid state, and is passed through a vaporizer prior toinjection into the settling chamber, wherein it mixes with and isthermally ionized by the high temperature gaseous jet, thereby producinga highly conductive plasma jet. The alkali metals, because of their lowionization potential, are preferred materials for use as ionizingagents.

Other objects and many of the attendant advantages of this inventionWill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconjunction with the solitary figure of the accompanying drawing whereinis shown a diagrammatic view, partially in section, of the apparatus ofthe present invention.

Referring now to the drawing, a metallic, thin walled, substantiallycylindrical chamber 11, in which the plasma is produced, is closed atits forward end by a transverse Wall 12 and terminates in an expansionnozzle 13 at its after end. A negative electrode or cathode 14 ispositioned in transverse wall 12 and extends into chamber 11 along thelongitudinal axis thereof. The forward cylindrical wall 15 of chamber 11serves as the positive electrode or anode and concentrically surroundscathode 14 as shown. Transverse wall 12 may be formed of suitableinsulating material so as to insulate the concentric electrodes 14-, 15from each other. A circuit l6, containing a direct current generator 17,is connected across cathode 14 and anode 15. The circuit 16 includes avariable resistance 13 by which the current flow through circuit 16 maybe selectively varied. Circuit 16 also includes therein a solenoidalcoil 19 wrapped about cylindrical anode 15 so as to produce an axialmagnetic fiel in chamber 11. This magnetic field spins the electricaldischarge or arc struck between electrodes l4, at a high speed; thuspreventing burnout of the anode l5, raising the energy input into thearc, and more uniformly heating a fluid, such as a compressed gas,passing through the are.

A high-pressure fluid source 21 is connected via conduit 23 and controlvalve 22 to chamber 11 so as to introduce a fluid flow into chamber lladjacent electrodes 14, 15 and through an electrical arc struck andmaintained between electrodes 14, 15. A preferred fluid for utilizationin this plasma generating apparatus is gaseous nitrogen, of which thephysical properties are to some extent similar to those of air. Thegaseous nitrogen having relatively large molecular size may be readily.

raised to high temperatures by an electrical arc. Also, the stability ofnitrogen at high temperatures limits the likelihood of the hightemperature, gaseous, nitrogen combining with the seeded alkali metals,as is discussed subsequently, thus enabling full use to be made of theits own resistance to current passing through circuit 29 connectedbetween chamber portion 26 and tube 27; A potential source 31 in circuit29 provides the energy for the heating current flow through circuit 29and tube 27. A piston member 32 is reciprocable in reservoir 24 to forcethe liquid ionizing agent 25 from reservoir 24 into vaporizing tube 27upon opening of valve 28. Other means, such as gas pressure, may be usedto force the liquid material 25 from reservoir 24; however it has beenfound that the use of a piston, as shown, best eliminates any pulsationsin the flow of theliquid material 25 from the reservoir.

The method of the invention maybest bedescribed with reference to theapparatus shown in the drawing, although it is to be understood that themethod may be performed with other apparatus as well.

The circuit 16 is first energized to provide an electrical discharge orare between electrodes'14, 1 5 in chamber 11; the coil 19 producing anaxial magnetic field in chamber 11.' The gas source 21 is then employedto introduce a gas fiow through conduit 23 into chamber 11 adjacentelectrodes 14, 15. The gas flow passes through the arc struck andmaintained between electrodes 14, 15, producing a high temperature,gaseous jet. This high temperature jet proceeds into after portion 26 ofcylindrical chamber 11 wherein it is seeded with the alkali metalionizing agent 25 and mixed therewith. The liquid alkali metal 25 inreservoir 24 is forcedtherefrom at a predetermined rate by piston 32 andenters the electrically heated vaporizing tube 27 through flow controlvalve 28. The liquid metal 25 is vaporized during its transit throughtube 27 and is then injected into the after portion 26 of chamber 11 toseed and mix with the high temperature gaseous jet. The chamber afterportion 26 serves as a settling chamber wherein the injected alkalimetal is evenly mixed with and ionized by the gaseous jet, and theplasma jet produced thereby is accorded a homogenous temperature andpressure distribution. The resulting plasma jet then issues from theexpansion nozzle 13 into an accelerator, not shown, or a laboratorytesting apparatus, not shown.

The above described seeding of a high temperature gaseous jet with analkali metal produces a highly conductive plasma jet, having an electronconcentration on the order of electrons per cubic centimeter. An alkalimetal is preferred as the seeding material or ionizing agent becauseofits low. ionization'potential; the vaporized alkali metal readilydissociating into positive ions and negative electrons when injected orfed into the high temperature, gaseous jet. The 'metal is vaporizedprior to being mixed with the gaseous jet'so that the heat available inthe jet may be used wholly to ionize the metal, and need not be expendedin vaporizing the liquid metal. The preferred alkali metalfor use in thesystem is cesium, dissociation energy 0.49 electron volt;

however, potassium and sodium have also been favorably employed; Aninteresting study on the dissociation properties of cesium may be foundin National Aero- 1 nautics and'Space Administration Technical NoteD-380,

published in May 1960, titled Temperature and Coms,201,ea5 I position'of a Plasma Obtained by Seeding a Cyanogen- Oxygen Flame with Cesium.

To afford a clearer understanding of the present invention, the physicalcharacteristics of one experimental apparatus constructed and operatedin accordance with this invention will now be described. A /2 inchdiameter water-cooled tungsten rod was-used for the cathode l4 andsurrounded by a 1 /8 inch diameter water-cooled copper cylinder for theanode 15. The circuit 16 for striking an are between electrodes 14-, wasoperated at a voltage of 110' volts and 1,400 amps, or approximately 150kilowatts. The efiiciency of the are as determined by calorimetric testswas 32 percent, which gives 1 and molecular nitrogen.

an enthalpy of 8,860 B.t.u./lb. and a stagnation temperature of -6,900K. The nitrogen from'source 21 was regulated to fiow at 2.6grams/second. The coil 19 wrapped about anode 15 produced an axialmagnetic field of 0.5 Weber/meter? The settling chamber 2 6 was made 2inches in length, it being found that a greater length resulted in arelatively larger heat loss to the chamber wall. a i

. The reservoir. 24 contained liquid cesium and was formed of Plexiglas.Piston 32 was motor-driven to force the cesium from the reservoir at aflow rate set to give a 2 percent mole-fraction ratio between the cesiumV The cesium was vaporized in a A; inch stainless-steel tube incheslong, electrically heated to 800 C. With the reservoir 24 containing 10grams of cesium, the duration of cesium flow was 40 seconds. Theresulting plasma emerged from nozzle 13 at a Mach number ofapproximately 2, a calculated static temperature of 5,500 IQ, and with across-sectional area of 1 square centimeter V s It is to be understoodthat the dimensions given and materials used in the above describedexemplary apparatus are merely illustrative of the present invention andthat the reduction or enlargement of the physical dimensions of theapparatus, or the substitution of other materials for those described,is well within the scope of the present invention and is contemplated bythe inventor thereof. 7 I

Obviously numerous modifications and variations of the present inventionare possible in the light of the above teachings. It'is therefore to beunderstood that within the scope of theappended claims the invention maybe practiced otherwise than as described herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. An apparatus for producing a highly conductive plasma jet comprising:means for striking and maintaina ing an electrical are; means forconveying a fluid to said electrical arc, thereby producing a hightemperature,

gaseous jet; a reservoir containing a material having aoutlet for thedischarge thereof; conduit means communicating said reservoir with saidsettling chamber; and means to force said ionizing material fromsaidreservoir through said conduit means into said settling chamber to mixwith and seed said jet, thereby increasing the conductivity of the jet.

- 2. An apparatus for producing a highly conductive plasma jetcomprising: means defining a substantially cylindrical chamber having aforward and an after end; means operatively connected thereto forstriking and maintaining an electrical arc across the forward end ofsaid chamber; means for conveying a fluid to said electrical are,thereby producing a high temperature, gaseous jet; a reservoircontaining a material having a low ionization potential; and means toconvey the ionizing materialfrom, said reservoir to the after portion ofsaid chamber to intermingle with and seed the high temperature, gaseousjet passing therethrough, thereby increasing the conductivity of thejet.

.5 5 3. An apparatus according to claim 2, wherein said FOREIGN PATENTSmeans to convey comprises a vaporizing tube connected 279 825 8/28 GmatBritain between said reservoir and the after portion of said chamher,and means for forcing the ionizing material from OTHER REFERENCES saidreservoir through said tube and into the after pOr- 5 H d et 1 NAsATechnical N t D480 M 17 tion of said chamber. 1950,

Rosa: R. 1., Shock Wave Spectroscopy and Engi- Eegerences Cum} by theExaminer neering Magnetohydrodynamics," thesis, Cornell Uni- UNITEDSTATES PATENTS versity, 1955, pages 30 to 45 relied upon.

3,014,154 12/61 Ehlers et a1 313231.5 X 10 V I 3,029,635 5/62 Feiz 313231.5 X GLGKGE N. WESTBY, Pumary Examlnei. 3,073,984 1/63 Eschenbach eta1. 313161 X 3,075,065 1/63 Ducatiet a1.

1. AN APPARATUS FOR PRODUCING A HIGHLY CONDUCTIVE PLASMA JET COMPRISING:MEANS FOR STRIKING AND MAINTAINING AN ELECTRICAL ARC; MEANS FORCONVEYING A FLUID TO SAID ELECTRICAL ARC, THEREBY PRODUCING A HIGHTEMPERATURE, GASEOUS JET; A RESERVOIR CONTAINING A MATERIAL HAVING A LOWIONIZATION POTENTIAL; A SETTLING CHAMBER POSITIONED ADJACENT SAID FIRSTNAMED MEANS AND HAVING AN INLET FOR THE INGRESS OF THE HIGH TEMPERATURE,GASEOUS JET AND AN OUTLET FOR THE DISCHARGE THEREOF; CONDUIT MEANSCOMMUNICATING SAID RESERVOIR WITH SAID SETTLING CHAMBER; AND MEANS TOFORCE SAID IONIZING MATERIAL FROM SAID RESERVOIR THROUGH SAID CONDUITMEANS INTO SAID SETTLING CHAMBER TO MIX WITH AND SEED SAID JET, THEREBYINCREASING THE CONDUCTIVITY OF THE JET.